1. Field of the Invention
The present invention relates to an encoder device, and more particularly to an encoder device for use in a focal length detector of a photographic zoom optical system in a camera or the like.
2. Related Art Statement
As is well known, a focal length detector of a photographing zoom optical system in a camera or the like employs a zoom encoder for detecting the amount and direction of rotation of a cam ring which is rotatable about an optical axis of the photographing optical system to change the focal length of the photographing optical system.
Conventionally, as is shown in FIG. 20, such an encoder for zooming etc. has been practiced by utilizing combinations of signals output upon contact pieces 308, formed of a conductor, sliding over patterns 312 which are made up by, for example, plating gold on the surface of an insulator 301.
In other words, to detect the rotational position of a cam ring which is rotated to carry out zooming etc., the illustrated encoder is provided with a code plate of several bits and brushes are brought into contact the code plate with for producing bit information from which the rotational position of the cam ring is detected. While this means for detecting the rotational position by using the code plate is advantageous in that the rotational position of the cam ring can be detected based on bit information from the code plate, it has disadvantages as set forth below:
1) the production cost of the code plate is high;
2) because of bit information being obtained from contact between the code plate and the brushes, oxidation of the code plate surface and the brush surface cause conduction failures with time; and
3) insufficient contact pressure, surface wearing, etc. also cause conduction failures with time.
Of these disadvantages, 2) and 3), i.e., the occurrence of failures with time, implies that such failures occur after products have been handed over to users and, therefore, are particularly quite critical problems from the standpoint of product quality.
For thoroughly solving the above disadvantages 2) and 3), therefore, it could be envisaged to replace the above-mentioned contact type encoder by a contactless absolute value encoder of the optical type using a photo-reflector etc. or the magnetic type using an MR element, etc.
Contrary to the intention, however, the above replacement means have substantial demerits in points of space and cost. These demerits become more severe as resolution of the zoom encoder is increased. The limit in practical use is on the order of 2 to 3 bits at maximum.
In view of the above, as disclosed in Japanese Utility Model Laid-Open No. 2-51310, there has been proposed such a means instead of the conventional contact type encoder that an optical pulse generator is provided which generates a pulse in response to rotation of a cam ring used for zooming etc., and the number of pulses output from the generator is counted up or down depending on the direction of rotation of the cam ring for zooming, to thereby determine the rotational position of the cam ring. This means enables a high-resolution encoder to be obtained by using only one photo-interrupter.
In such a zoom encoder using a photo-interrupter, a rotating plate with a slit and a corresponding photo-interrupter are arranged midway a train of reduction gears leading from a zoom motor to the cam ring, the rotating plate with the slit is rotated with rotation of the zoom motor, and a signal generated from the photo-interrupter is used to determine the rotational position of the cam ring. For obtaining a high-accuracy (high-density) signal from the above encoder, the photo-interrupter is required to be arranged near the first stage of the reduction gear train, i.e., in a portion rotating at a high speed. However, because the reduction gear train includes many speed reducing gears from the first stage to the output gear stage, there essentially occur large amounts of backlash. Stated otherwise, since large amounts of backlash are present between the photo-interrupter arranged near the first stage and the cam ring, there is no linear relation between an output of the photo-interrupter and the cam ring, meaning the presence of problems in accuracy.